This is a Eurorack version of the two_comparator_effect originally designed by Rob Schafer and later improved by cyberboy666. I communicated with cyberboy666 before publishing this module and received his blessing to do so.
The circuit compares the voltage of a composite video signal to two different threshold voltages to produce two binary signals. That is, if the video signal is brighter than the threshold, the output is "high" or white. If the video signal is darker than the threshold, the output is "low" or black. One of the binary signals is sent out at a lower voltage level than the other, so the mixed output of both signals is white, gray and black.
A demo video is available if you would like to see the module in action. The video shows a version that used tall trimmer pots instead of the more robust Alpha 9mm pots in the BOM, but it is otherwise identical.
It's worth noting that this circuit uses an LM339 comparator, which is not quite fast enough for video. The LM339 has a response time of about 1.3 microseconds, and we really need a response time below 83 nanoseconds for video. As a result, the resulting video image is slightly shifted to the right. This is not an issue unless your are mixing the output with the original signal in some way. Even then, the offset sort of looks interesting. I will say this for the LM339: it's very cheap, easy to find, and it comes in a quad through-hole package. Unfortunately, with the way through-hole components are going extinct, there aren't a lot of other great options in a quad package.
When soldering, please also see the interactive BOM. After soldering a component, make sure to trim the excess leads as much as possible. This is particularly important where the potentiometers sit over the top of solder pads for other components.
Steps:
- Solder all resistors and the D1 diode on the primary, back side of the circuit board (the side with the open source hardware logo). I call this the back side because it faces away from the front panel. D1 says 1N914 on the BOM but this can be replaced by a 1N4148 or 1N5819. I use a 1N4148 in all of my builds. Make sure to get the polarity correct when you place diodes! The line on the circuit board should match the stripe on the diode.
- Solder the diodes and ferrite beads on the front side of the board. D2 and D3 are the standard Eurorack reverse polarity protection diodes. I use 1N4004, but you can use any diode in the 1N400x series. Once again, do make sure to get the polarity correct when you place the diodes.
- Solder the ceramic capacitors on the back side of the board.
- Solder the socket for the IC, if you want to one. People say it introduces extra capacitance, but it's probably no big deal in this circuit. A socket will protect the IC and make it reusable in other projects.
- Solder the transistor and voltage regulators (U2, U3, Q1). Make sure they are facing the correct direction as indicated on the circuit board.
- Solder the power headers. Please note that a shrouded, keyed power header will be very slightly too wide for a 4hp module (fractions of a millimeter) so you should probably use normal header pins here. The PCB is marked to indicate where the -12v (red stripe) of the power cable should go.
- Solder the electrolytic capacitors. Mind the polarity -- the white side of the circle on the circuit board is the negative side of the capacitor.
- Place but do not solder the jacks, knobs and switch on the front side of circuit board.
- Place the front panel over the jacks, knobs and switch, and then use their nuts to attach them to the front panel.
- Solder the jacks, knobs and switch.
- If you used an IC socket, put the IC in the socket. If you did not use a socket, solder the IC now. Mind the polarity -- the small semi-circle on the board should match the notch on the IC.
- Test and enjoy!
